Talks and Presentations Slides of most talks since 2002 in reverse chronological order. No guarantees; always out of date; some results might be good and just not published yet, some might be wrong. Minimum copy-protection, but I do ask that you cite if you steal anything. Some talks are not posted yet because they include yet to be published results (spin and orbital quantum dot relaxation, valley qubit stuff, etc., coming very soon). Technical Talks General Audience Guest Lectures SILICON IN THE QUANTUM LIMIT:QUANTUM COMPUTING AND DECOHERENCE IN SILICON ARCHITECTURES audience/type/location: technical; Ph.D. Thesis Defense; Madison, WI speakers: Charles Tahan given: August 17 , 2005 length: 60 minutes abstract.: The pursuit of spin and quantum entanglement-based devices in solid-state systems has become a global endeavor. The approach of the quantum size limit in computer electronics, the many recent advances in nanofabrication, and the rediscovery that information is physical (and thus based on quantum physics) have started a worldwide race to understand and control quantum systems in a coherent and useful way. Semiconductor architectures hold promise for quantum information processing (QIP) applications due to their large industrial base and perceived scalability potential. Electron spins in silicon in particular may be an excellent architecture for QIP and also for spin electronics (spintronics) applications. While the charge of an electron is easily manipulated by charged gates, the spin degree of freedom is well isolated from charge fluctuations. This leads to very good spin quantum bit (qubit) stability or quantum coherence properties. Inherently small spin-orbit coupling and the existence of a spin-zero Si isotope also facilitate long single spin coherence times. Here we consider the relaxation properties of localized electronic states in silicon due to donors, quantum wells, and quantum dots. Our analysis is impeded by the complicated, many-valley band structure of silicon and previously unaddressed physics in silicon quantum wells. We find that electron spins in silicon and especially strained silicon have excellent decoherence properties. Where possible we compare with experiment to test our theories. We go beyond issues of coherence in a quantum computer to problems of control and measurement. Precisely what makes spin relaxation so long in semiconductor architectures makes spin measurement so difficult. To address this, we propose a new scheme for spin readout which has the added benefit of automatic spin initialization, a vital component to quantum computing and quantum error correction. Our results represent important practical milestones on the way to the design and construction of a silicon-based quantum computer. PDF: soon Long-lived spin and valley states in lateral silicon quantum dots for quantum information processing audience/type/location: technical; contributed; APS March Meeting, Los Angelos, CA speakers: Charles Tahan given: March 22, 2005 length: 10 minutes abstract.: Quantum electronics in silicon offers both challenges and opportunities for information technology. Here we take a challenge, the doubly degenerate conduction band minima of silicon quantum wells, and make it into an opportunity, stable valley states for quantum storage and control. We calculate the coupling between the two valley states using both tight-binding and approximate-analytic techniques for a lateral quantum dot. This determines the valley relaxation and optical excitation rates. Not only are the relaxation times uncharacteristically long for excited orbital states, but we find that for finite quantum wells there are 'magic' electric fields where the coupling goes to zero, suppressing valley relaxation and excitation in first order. In the process we derive new expressions for single-valley orbital relaxation (which is very fast) and qubit spin-flip times (due to spin-orbit coupling) and compare them to valley state relaxation. We discuss important implications for 'valley qubits' in silicon quantum information processing and technology. PDF: soon Nanotechnology and Society audience: non-technical; contributed; APS March Meeting, Los Angelos, CA speakers: Charles Tahan given: March 23, 2005 length: 10 minutes abstract.: Nanotechnology has emerged as a broad and exciting, yet ill-defined, field of scientific research and technological innovation. Important questions have arisen about the technology's potential economic, social, and environmental implications by prominent technology leaders, nanotechnology boosters, science fiction authors, policy officials, and environmental organizations. We have developed a freshman-level seminar course that offers an opportunity for students from a wide range of disciplines, including the natural and social sciences, humanities, and engineering, to learn about nanoscience and nanotechnology and to explore these questions and reflect on the broader place of technology in modern societies. The course is built around active learning methods and seeks to develop the students' critical thinking and research skills, written and verbal communication abilities, and general knowledge of nanotech. Continuous assessment is used to gain information about how effective the class discussions are and how well the overall course enhances students' understanding of the interaction between nanotechnology and society. * In conjunction with: W.C. Crone, Engineering Physics, K.D. Ellison, Graduate School, R. Leung, Sociology, C.A. Miller, Science and Technology Studies, and G.M. Zenner, Materials Research Science and Engineering Center PDF: cgtNanosocMarch2005.pdf The Ambassadors of Nano audience: non-technical; invited; Fall 2004 Delta Forum (University of Wisconsin-Madison) speakers: Charles Tahan given: December 16, 2004 length: 15 minutes abstract.: Nanotechnology has emerged as a broad and exciting, yet ill-defined, field of scientific research and technological innovation. Important questions have arisen about the technology's potential economic, social, and environmental implications by prominent technology leaders, nanotechnology boosters, science fiction authors, policy officials, and environmental organizations. We have developed a freshman-level seminar course for next semester that offers an opportunity for students from a wide range of disciplines, including the natural and social sciences, humanities, and engineering, to learn about nanoscience and nanotechnology and to explore these questions and reflect on the broader place of technology in modern societies. The course is built around active learning methods and seeks to develop the students' critical thinking and research skills, written and verbal communication abilities, and general knowledge of nanotech. Continuous assessment will be used to gain information about how effective the class discussions are and how well the overall course enhances students' understanding of the interaction between nanotechnology and society. In preparation we have also given several guest lectures this semester for undergraduate courses across campus and led a graduate seminar on Nanotechnology and Society. * In conjunction with: W.C. Crone, Engineering Physics, K.D. Ellison, Graduate School, R. Leung, Sociology, C.A. Miller, Science and Technology Studies, and G.M. Zenner, Materials Research Science and Engineering Center PDF: cgtNanoSocDelta.pdf Nanotechnology, Medicine, & the Body audience: educational/non-technical; invited; Medical History and Bioethics, University of Wisconsin-Madison speakers: Charles Tahan, Ricky Leung given: December 2, 2004 length: 50 minutes abstract.: Introduction to nanotechnology and society in regards to body modification. PDF: cgtNanoMedIntro.pdf Nanotechnology, Society, & the Engineer audience: educational/non-technical; invited; Intro. to Engineering 160, University of Wisconsin-Madison (Eng. Dept.) speakers: Charles Tahan, Ricky Leung, Wendy Crone given: November 17 & 18, 2004 length: 80 minutes abstract.: Introduction to nanotechnology and society for freshman engineering design class. PDF: cgtIntroToEng.pdf Quantum dot quantum computers and other entanglement-based devices audience: technical/educational; invited; Physical Chemistry Seminar, University of Wisconsin-Madison, Chemistry Dept. speakers: Charles Tahan given: November 9, 2004 length: 50 minutes abstract.: Information is physical. This rediscovery has lead to a paradigm shift in the fields of information theory and computer science. Perhaps the laws of nature, rather than the abstract world of 0s, 1s, and Turing machines, provide a more fundamental foundation for computing? This thesis lead to the discovery of quantum algorithms and quantum communication schemes which, in some cases, are more powerful than their classical counterparts. The key resource is quantum entanglement, the non-local interactions pervasive in quantum theory. Meanwhile, physicists are reaching the nanoscale limit, where dynamical control of quantum systems has, for the first time, become a possibility. My perspective on this is that of an electron spin trapped in silicon. Despite the complexities, this quantum bit may be a great place to store and manipulate quantum information. I’m going to try and explain entanglement via a few physical situations for spin-1/2 particles, nature’s natural qubits. Quantum states are fragile and must remain coherent long enough to do a computation. Here, quantum error correction schemes come into play. Of course, I’ll also talk about the quantum computing architecture envisioned and currently being pursued by some of us in Madison. PDF: cgtChemistry2004.pdf Spintronics and spin-based quantum dot quantum computing audience: educational/technical; guest lecture; ECE746-Quantum Electronics (R. Blick), University of Wisconsin-Madison (ECE Dept.) speakers: Charles Tahan given: October 26, 2004 length: 80 minutes abstract.: Guest lecture on basics of quantum computing and quantum dot quantum computing for Robert Blick's quantum electronics class. Slightly updated over 2004 version with inclusion of some spintronics and the Datta-Das device. PDF: cgtBlickQDQC2004.pdf Spin-Based Quantum Dot Quantum Computing in Silicon audience: technical; invited; Cavendish Laboratory, University of Cambridge, UK speakers: Charles Tahan given: October 12 , 2004 length: 50 minutes abstract.: Review of Wisconsin solid-state quantum computing group's progress on QDQC in silicon (from my bias). PDF: soon Spin and pseudo-spin states in silicon for QC: lifetimes audience: technical; contributed; APS March Meeting (Montreal, CAN) speakers: Charles Tahan given: March 28, 2004 length: 10 minutes abstract.: Silicon devices are special among few-electron quantum nanodevices because of the many-valley nature of the silicon conduction band. In addition to spin and orbital states, valley states are formed from linear combinations of valley minima. In quantum well quantum dots, these valley states are split in energy by confinement and, it turns out, can be long-lived. We calculate the lifetimes of these valley pseudo-spin states and consider their relevance in quantum information processing. Experimental signatures of such states will also be discussed. PDF: cgtMarch2004.pdf Signatures of Silicon: Spin and Valley States audience: technical; contributed; Solid State Quantum Information Processing Conference (Amsterdam, NL) speakers: Charles Tahan given: December 15-18, 2003 length: poster abstract.: Silicon architectures for quantum information processing (QIP) have many promising attributes over other semiconductor materials but also several complexities. We explore some of these differences by analyzing the spin relaxation times of potential electron spin qubits in both gated quantum dots and quantum well two-dimensional electron gases, characterizing spin-orbit coupling in such layered heterostructures along the way. Further, we calculate transition matrix elements to nearby valley-split states (unique to silicon) and the corresponding phonon and photon transition rates. Valley state lifetimes are found to be long. We postulate on uses for these unique valley pseudo-spin states for QIP and signatures of such states in ESR, transport, and Shubnikov De-Haas experimental measurements. To my knowledge this is the first introduction of the idea of valley qubits in the community. PDF: AmsterdamPoster.pdf Spin-based quantum dot quantum computing: the ultimate in quantum electronics audience: educational/technical; guest lecture; ECE746-Quantum Electronics (R. Blick), University of Wisconsin-Madison (ECE Dept.) speakers: Charles Tahan given: October 9, 2003 length: 80 minutes abstract.: Guest lecture on basics of quantum computing and quantum dot quantum computing for Robert Blick's quantum electronics class. PDF: cgtBlickQDQC2003.pdf Amazing feats with artificial atoms: single spin readout and fast initialization audience: technical; invited; Atomic Physics Seminar, University of Wisconsin-Madison, Physics Dept. speakers: Charles Tahan given: September 9, 2003 length: 50 minutes abstract.: We've devised a scheme for the measurement of a single electron spin in a semiconductor quantum dot. It utilizes microwaves (i.e. light, in atomic lingo) to induce spin-dependent charge motion which can be detected by a sensitive electrometer. The moral equivalent of optical pumping is used to polarize the electron in the process. Thus, readout and initialization of a spin-based qubit in a quantum dot quantum computer can be achieved. Progress has been made on actually building such a device here in Madison. The question is, will it work? PDF: cgtAtomic2003.pdf Spin Readout and Initialization in a Semiconductor Quantum Dot audience: technical; contributed; 2nd International Workshop on Quantum Dots for Quantum Computing and Classical Size Effects Circuits, Notre Dame, IN speakers: Charles Tahan given: August 9, 2003 length: 20 minutes abstract.: Electron spin qubits in semiconductors are attractive from the viewpoint of low environmental coupling and long coherence times. However, both spin readout and efficient qubit initialization remain significant challenges for quantum dot quantum computing. Unfortunately, promising schemes based on spin-charge transduction introduce external couplings in the form of reference qubits or Coulomb blockade leads. Here, we propose a twist on the spin-charge transduction scheme, converting spin information to orbital information within a single quantum dot (QD). The same QD can be used for initialization, gating, and readout, without unnecessary external couplings. We present detailed investigations into such a scheme in both SiGe and GaAs systems: simulations, including capacitive coupling to a RF-SET, calculations of coherent oscillation times which determine the read-out speed, and calculations of electron spin relaxation times which determine the initialization speed. We find that both initialization and readout can be performed within the same architecture. Work supported by NSF-QuBIC and MRSEC programs, ARDA, and NSA. PDF: cgtIWQDQC2003.pdf Spin-flip transitions in silicon quantum dots audience: technical; Doctoral Prelim Exam Talk; University of Wisconsin-Madison (Physics) speakers: Charles Tahan given: Spring, 2003 length: 50 minutes abstract.: Prelim exam: theory for spin relaxation of donors in strained silicon, orbital and spin relaxation in silicon quantum well quantum dots, and of the Wisconsin readout scheme utilizing optically-induced spin-charge transduction for readout and initialization. PDF: cgtPrelim2003.pdf Single Qubit Spin Readout and Initialization in a Quantum Dot Quantum Computer: Design and Simulation audience: technical; contributed; APS March Meeting (Austin, TX) speakers: Charles Tahan given: March 7, 2003 length: 10 minutes abstract.: Introduction to Wisconsin's scheme for optically-induced spin-charge transduction. PDF: cgtMarch2003.pdf What's a Quantum Computer, Charlie? audience: general; Graduate Student Council, Wisconsin-Madison authors: Charles Tahan given: Fall 2002 length: 10-15 minutes abstract.: Quantum computers don't exist yet. But I exist, and I can study anything I want. As a would-be theoretical physicist, quantum computing is a wonderful idea - mainly because it's very cool (= funding) and quite useless at the moment (= good research). There's actually a big group in Madison trying to build a quantum computer. I'd like to tell you a little about it. What's a quantum computer? The real question is: Can I answer this question without causing that glazed over look that usually appears when I start talking about what I actually do? We'll see. (Legal notice: Many trademarks will be violated in this talk.) PDF: charlieQC.pdf Growth Simulations of Single Crystal Perovskite Alloys audience: technical; Undergraduate Honors Thesis Defense; College of William and Mary, VA (Physics Dept.) speakers: Charles Tahan given: May 3, 2000 length: 50 minutes abstract.: We study the growth process of ferroelectric materials by kinetic Monte Carlo simulations. An ionic model with large-range Coulomb interactions is used to model the relaxor single crystals. The growth is characterized by thermodynamic processes involving adsorption and evaporation with solid-on-solid restrictions. An algorithm is developed in order to simulate growth under such a model for which existing formalism of the kinetic Monte Carlo algorithm is inadequate. We study the growth rates and the order structure of the grown crystals as a function of temperature, chemical composition, and growth orientation. Tests of our algorithms on NaCl give good results. PDF: cgtHonors2000.pdf